Supermarket giant Coles has launched a surprise attack on its rival Woolworths by secretly buying one of its most profitable stores on Sydney's lower north shore.

Woolworths has confirmed that Coles has recently purchased its highly profitable stand-alone store in Neutral Bay.

A Woolworths spokesman says the sale came as a surprise for the company.

But it is not an unprecedented move - Coles was forced to find another location after Woolworths bought its Katoomba supermarket in the Blue Mountains late last year.

Matthew Levey from consumer group Choice says despite the real estate jostling, customers will not be left out of pocket.

"People tend to shop on the basis of convenience and price," he said.

"I'm not sure of the degree consumers differentiate between the two beyond what's convenient, but certainly the two big supermarkets keep getting bigger and competing with each other in some quite interesting ways."

The Australian Competition and Consumer Commission is already investigating allegations Coles and Woolworths bullied suppliers and discriminated in favour of their own generic products.